Ancient Ailments

Squirrels were considered the height of fashion in medieval Europe. Anyone who could afford to do so would have a coat made from the fur of these small rodents. For example, there were some years in which the English royal court ordered thousands of coats of squirrel fur so that people could stay warm and dress in a way that befitted their social status. This gave rise to a real boom in the fur trade, which extended right across Europe and stretched as far as Russia. Keeping a squirrel as a pet was also very much in vogue in esteemed circles. The French Queen Isabeau de Bavière, who hailed from Bavaria and lived from 1370 to 1435, shared her royal chambers with one such furry animal that wore a specially made collar bedecked in pearls. “Back in this period of history, squirrels and people were undoubtedly much closer companions than they are today, when we might simply encounter the animals in the woods from time to time,” says Verena Schünemann. Although keeping a pet squirrel may sound rather comical, this co-existence might not have been as harmless as it seems. Schünemann’s research clearly illustrates why. 

Verena Schünemann is a paleogeneticist. One of the topics she is researching in the URPP Evolution in Action at the University of Zurich is how the great plagues in history, which today we would tend to refer to as pandemics, actually originated and how the pathogens evolved over time and constantly adapted to human beings. This is because the agents that cause diseases such as leprosy, the plague or syphilis are intractable foes. They have been around for centuries, causing millions of deaths – and are still doing so to this day.

In medieval squirrel bones that were discovered in the English city of Winchester, Verena Schünemann and her team have now identified traces of Mycobacterium leprae, which is the bacterium that causes leprosy. This is the same germ that the paleogeneticist has also detected in human remains from the Middle Ages. “It’s entirely possible that in the past this pathogen circulated between human beings and squirrels,” says the researcher. However, it isn’t yet clear whether and in which direction any transmission took place – so whether animals infected humans or rather humans infected animals. But what is certain is that leprosy was rife in medieval Europe. The spread of the disease was most prevalent in the 13th century, before the leprosy wave subsided toward the end of the 16th century. 

Infection with the leprosy pathogen results in pathological changes to the skin, mucous membranes, nerve tissue and bones and physical disfigurement that can be seen in pictures of victims of the disease. The bacterium itself is not usually fatal, but the subsequent infections that may be facilitated by changes to the tissue can be. Even today, some 200,000 people around the world are still infected with leprosy every year – mostly in India, South-East Asia and South America. Analyses that Verena Schünemann has conducted with other researchers show that the genome of today’s leprosy pathogens is still largely identical to that of pathogens originating from the Middle Ages. This means that the genetic material of this pathogenic bacterium has barely changed over many centuries. “The pathogen has built up an effective defense mechanism as it has evolved,” says Verena Schünemann, “and this is why treatment with antibiotics is still very protracted and tough even today.”

Whether leprosy originated from a zoonosis, which is when a pathogenic agent makes the jump from animals to humans, is still unclear. But what is clear is that zoonoses play a key role in the development of many “classic” infectious diseases. The best-researched example is the plague, which raged throughout Europe for centuries and had a devastating impact. The plague pathogen, Yersinia pestis, is transmitted primarily by rats from fleabites, and killed millions of people in the Middle Ages and early modern period.

Back in 2011, Verena Schünemann together with Johannes Krause, who was her boss at the University of Tübingen at this time, managed to decipher the genome of the medieval plague pathogen by conducting DNA analyses on the remains of plague victims buried in a London cemetery. This enabled the researchers to compare the historical pathogen with today’s plague pathogens and chart its evolution to the present day.

The Black Death has not yet been eradicated globally. Every year, hundreds of people still contract the plague in global hotspots like Kazakhstan. “In the Yosemite National Park in the United States, there are signs advising visitors to avoid contact with chipmunks to prevent possible infection,” says Verena Schünemann, “because these animals could carry the plague pathogen.” The chipmunks are potential “reservoirs” in which germs can propagate in the absence of humans.

These animal reservoirs could be a key reason why the great pandemic diseases of yesteryear have managed to survive to the present day. “The host range for pathogens like the plague pathogen is vast; it can be hosted in many different rodents, for example, and from there it is capable of making the jump to humans,” says paleogeneticist Schünemann, “and this ability to infect different hosts is a key element of this bacterium’s survival strategy.” That’s why to get rid of the plague it would theoretically be necessary to eradicate the plague from all rodent populations around the world, which is virtually impossible. This is the reason why we’ll probably have to continue living alongside this infectious disease in the future, albeit with less severe consequences than the impact it had in the Middle Ages and early modern period. The historical role that rodent reservoirs may have played in the spread and evolution of the plague and leprosy is set to be analyzed in more detail by Verena Schünemann, who has now taken up a professorship at the University of Basel, with an EU grant that she was awarded last year.

Although leprosy and the plague have not disappeared, antibiotics mean they are now nowhere near as dangerous as they used to be. On the other hand, there is currently a real resurgence in another infectious disease that has affected human beings for a long time, namely syphilis. “The COVID-19 pandemic in particular provided a real boost to the disease,” says Verena Schünemann. Scientists are unable to explain exactly why this is. The fact is that syphilis infections are rising steadily across the globe.

Syphilis is one of many treponematoses – infectious diseases that are triggered by Treponema bacteria. While syphilis is a sexually transmitted disease, other treponematoses such as yaws or endemic syphilis are passed on mainly through smear infections between people living together in close relationships. The syphilis bacteria responsible for the current numbers of infections are transmitted through sexual contact. The focus is on very similar pathogens around the world. “They belong to a specific substrain of the traditional syphilis strains,” says the researcher. Syphilis and other treponematoses can be treated with antibiotics like penicillin. If the disease remains untreated, it can cause damage to organs and the nervous system, produce ulcers all over the body and ultimately prove fatal. 

There are differing views within the scientific community about where syphilis originated historically and how the disease spread in Europe in the past. A theory advocated by many researchers is that Christopher Columbus and his crew brought the classic syphilis pathogen home with them from the New World in 1493. The infectious disease spread in the late 15th century in particular from ports right across Europe. However, using various analyses of historical and prehistorical DNA samples from human bones, Verena Schünemann and her team have been able to show that the syphilis story is much more complex than was previously assumed and other subtypes like the pathogens of endemic syphilis also need to be considered, not just classic syphilis. 

Verena Schünemann’s research is not just producing new, sometimes surprising findings in relation to the spread and evolution of infectious diseases, it is also refreshing our memory of pandemics. For even though we occasionally forget this, there has been a constant battle between humans and microbes throughout history. “When we look to the past, it’s clear that there have always been pandemics. That’s why we should now also always be prepared for the next one to occur,” says Schünemann.

The paleogeneticist is in no doubt that we will have to contend with new pandemics in the future, especially as our modern way of life encourages the development of infectious diseases. In a globalized world in which megacities are expanding, people are living in ever closer proximity – and this also makes it easier for pathogens to be transmitted. “The same is true of intensive livestock farming in which infections can spread rapidly,” says Verena Schünemann. The paleogeneticist’s research also clearly illustrates how important animals have been as reservoirs for pathogens throughout history and in the evolution and spread of infectious diseases. “That’s why the international monitoring of wild animals, which has been stepped up since the COVID-19 pandemic, should definitely be maintained,” says Schünemann.

This article is part of the  UZH Magazin «Kostbare Vielfalt»